1. Field of the Invention
This invention relates to a semiconductor laser beam splitting device, and more particularly to a beam splitting device for splitting a laser beam generated from a semiconductor laser beam precisely in a desired proportion irrespective of the amount of light.
2. Description of the Prior Art
A semiconductor laser having a semiconductor laser chip has been used as a scanning light generating means for various scanning recording systems, scanning reading systems and the like. As compared with a gas laser, the semiconductor laser is advantageous in that it is small in size, is inexpensive and has low power consumption, and in that its output can be changed by controlling the driving current (analog direct modulation). Especially for the scanning recording system, the semiconductor laser is very useful since its output can be directly modulated in accordance with signals bearing image information.
However, the semiconductor laser generates both laser light and spontaneous emission, and this fact gives rise to the following difficulty. That is, as can be seen from FIG. 6, line a represents the relation between the electric current imparted to the semiconductor laser and the spontaneous emission output of the semiconductor laser and line be represents the relation between the electric current imparted to the semiconductor laser and the laser light output when the electric current imparted to the semiconductor laser is smaller than a threshold value Io, only spontaneous emission is output from the laser and no laser light is output. Though spontaneous emission output gradually increases with increase in the driving current, the proportion of spontaneous emission output to the total output becomes very small as the driving current exceeds the threshold value Io so that laser light begins to be output and laser light output increases. The spontaneous emission output finally comes to be negligible. The relation between the driving current and the total output is represented by line c in FIG. 6. In order to use the semiconductor laser in the recording system, the reading system and the like described above, it is sometimes preferred that the amount of light generated from the laser be monitored from outside. When monitoring the amount of light generated from a light source, generally a beam splitting element such as a beam splitter is positioned on the optical path of the light generated from the light source to divide the light into two light beams in a desired proportion, and a photodetector is provided to detect the amount of light of one of the two light beams. However, in order to monitor the amount of light of one of the divided light beams by way of the amount of light of the other divided light beam, the light beam generated from the light source must be divided in a fixed proportion irrespective of the amount of light of the light beam. In the case of the light beam generated from the semiconductor laser, the proportion of the laser light to the spontaneous emission changes according to the total amount of light generated therefrom as described above. Since the laser light substantially solely consists of light linearly polarized in the direction parallel to the semiconductor juncture of the semiconductor laser chip while the spontaneous emission includes both light polarized in the direction parallel to the junction and light polarized in the direction normal to the junction in equal proportion, the polarized light components contained in light impinging upon the beam splitting element varies depending on the proportion of the laser light component to the spontaneous emission component which differs with the output range of the semiconductor laser. The polarized light components contained in light impinging upon the beam splitting element affects the division of light by the beam splitting element, and accordingly, the amount of light generated from the semiconductor laser cannot be correctly monitored by conventional beam splitting optical systems.